HARVEY'S WORK 277 



reasoned hypotheses. Four years after Harvey's death the great 

 Italian anatomist Malpighi saw under the microscope these 

 capillaries which the physician had seen with the eye of faith. 

 The demonstration of the actual passage of blood from arteries 

 to veins through capillary channels was given in 1688 by Leeuwen- 

 hoek, the illiterate janitor of the aldermen of Delft. 



Dynamically considered, the blood acts in much the same way 

 as any other equally viscous fluid driven through a series of tubes. 

 In order to understand many of the problems which one meets 

 in the study of physiological phenomena, it is necessary to obtain 

 some insight into the movement of fluid under an external driving 

 force. As Servetus says, ' ; In order to learn how the blood is 

 formed it is necessary to ascertain how it moves." First of all, 

 let us consider the flow of liquid from a reservoir through a series 

 of tubes. In a liquid the molecular forces are in equilibrium ; 

 the kinetic forces characteristic of matter in the gaseous state are 

 exactly balanced by the Newtonian forces predominant in solids. 

 As Soddy would put it, the processes of pellalion and tractation 

 would not be manifest. Gravitation alone has to be reckoned 

 with. In common parlance, liquids seek their own level and so 

 always tend to flow to the lowest possible position. It is a well- 

 known fact that the speed attained by a body falling in vacuo 

 through the distance (h) equals \/2gh, g being the acceleration 

 produced by gravity. This formula cannot be used to estimate 

 the velocity of fluid escaping from a reservoir. As every boy 

 knows, when the waste water is being run out from the bottom 

 of a wash-hand basin, the fluid tends to rotate round the orifice 

 and to assume a conical form. This is due to the attempt of the 

 water particles to rush the exit (so to speak). Only a limited 

 number of them lie in the column vertically above the opening. 

 The majority, occupying more lateral positions, tend to escape 

 along with the minority in the queue and so exert a force applied 

 at an angle to the line of exit. Consequently, the total energy 

 cannot be used to produce velocity. Some of it has to be spent 

 in overcoming the resistance at the outlet. 



Still further modification of the formula is required if the 

 orifice is fitted with an exit tube. It must be evident that the 

 presence of this passage imposes a greater resistance to outflow 

 and materially reduces the rate. Let us consider the effect 

 produced on rate of flow by attaching a rigid cylindrical tube of 

 uniform bore to the lower orifice of the reservoir. In order to 

 simplify matters, we will place this pipe horizontally. Two causes 



